The Boxing Day tsunami has several predecessors, scientists have found. Two papers in this week's Nature by Geoscience Australia researcher Amy Prendergast and team, and Kent State University Ohio scientist Katrin Monecke and colleagues have found evidence for at up to three previous massive tsunami events in the Indian Ocean. Both teams made their discoveries by taking core samples from marshy ridges behind the modern-day beaches in parts of Thailand and Sumatra respectively. The theory was fairly straight forward. Sudden large in-shore movements of water carry with them large amounts of sand and gravel which are then deposited uniformly wherever the water goes, covering the ground like a thin layer of icing. Over time these deposits are covered by soil and other materials as the flooded area recovers and returns to normal. But the gravel layers remain like "tsunami tide-marks" for centuries and can be pinpointed in time by carbon-dating material buried within the deposits and by other geological techniques such as looking at the rate at which the beach ridges have advanced. The teams found deposits consistent with up to three previous tsunamis including one likely to have happened in 1907 and for which there is a strong historical record. The other two occurred at around 700 years ago and the third about 1300 years ago. At the two sampling locations studied by the researchers the waves would have been up to 35 metres high when these events occurred. Previously there was thought to be no historical precedent for the 2004 tsunami; now we know differently. But what's still an issue, the researchers point out, is what this means to the people of Sumatra and elsewhere who live beside this threat.